[SPARK-15894][SQL][DOC] Update docs for controlling #partitions

## What changes were proposed in this pull request?
Update docs for two parameters `spark.sql.files.maxPartitionBytes` and `spark.sql.files.openCostInBytes ` in Other Configuration Options.

## How was this patch tested?
N/A

Author: Takeshi YAMAMURO <linguin.m.s@gmail.com>

Closes #13797 from maropu/SPARK-15894-2.